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Numerical estimation of maximum possible sizes of paleo-earthquakes and tsunamis from storm-derived boulders

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Abstract

Estimating the magnitudes of potential large earthquakes and associated tsunami hazards along a subduction zone is important for risk evaluation. Coastal boulder deposits can serve as valuable geological indicators in intertidal zones with high wave velocity, even though finer sediments are generally washed away. This report presents a new method to constrain the maximum possible sizes of paleo-earthquakes and tsunamis using boulders of storm wave origin as a constraint in numerical simulations. The fundamental assumption is that presently observed storm-derived boulders have not moved a great distance by past tsunami waves. This illustrative case study uses the sizes and spatial distributions of storm wave boulders on the reef at Kudaka Island, Okinawa Prefecture, which faces the Ryukyu Trench. In all, 47 tsunami scenarios with different locations and fault parameters were assumed. Results show that tsunami waves with a maximum water level of 2.2 m and maximum flow speed of 1.5 m/s at a beach of Kudaka Island did not strike the island. Under this condition, earthquakes of Mw≥8.3 are unlikely to have occurred in the central Ryukyu Trench if one obeys an empirical scaling law for subduction earthquakes or ≥2 m of maximum possible slip near the trench if the fault size is fixed as 100 km × 50 km. Those tsunami and earthquake sizes could not have been generated during this period if one assumes that the reef of Kudaka Island and emplacement of storm boulders began more than 3500 years ago. This finding implies that the seismogenic mechanism of the Ryukyu Trench and the risk of a giant earthquake differ from those of other trenches such as the Japan Trench and the Chilean Trench, where giant earthquakes of Mw≥9.0 have occurred repeatedly during the past. The proposed method would enable researchers to estimate the maximum possible sizes of the paleo-earthquakes and tsunami waves in any region, even in regions with a vague or unknown history of past events because of a lack of sandy tsunami deposits or seismic records.

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This paper presents data and analysis for block and boulder transport during Super Typhoon Haiyan along a 4.5km long, low (5-12m) cliffed coastline in Calicoan Island, Eastern Samar, Philippines. Wave runup exceeding 15.2m elevation above mean sea level drove large limestone clasts, with volumes up to ~ 83m³, up to ~ 280m inland. A few very large clasts with volumes 65-132m³ were not transported by the waves. When combined with recent transport reported in May et al. (2015), Cox et al. (2016), and other literature, it is becoming increasingly clear that the largest blocks transported by storms overlie much of the tsunami transport range, increasing the difficulty in attributing the transport source without additional evidence. Comparison of present results with a global database of storm boulder transport shows a mass-elevation envelope outside of which no transport is observed.
Article
Holocene coral reefs are formed over equatorial pressure belts, trade wind belts, and subtropical high-pressure belts (between latitude 30° N and 30° S). In equatorial pressure belt and trade wind belt regions, few coral reefs are affected by storm-induced waves. In subtropical high-pressure belts, on the contrary, sediments on coral reefs are transported by catastrophic waves generated by violent storms (typhoons, hurricanes and cyclones). The wind belts could influence the geomorphic processes of coral reefs, resulting in different landforms. Several typhoons hit the Ryukyu Islands located in southwestern Japan every summer, and their coral reefs have sustained successive damages before recovering. Large blocks (more than 1 m in longest diameter) detached from reef slopes are found on reef flats in some coral reefs. They can move during catastrophic disturbance, and may be transported by storm-induced waves. The object of this study is therefore to evaluate the effect of large reef blocks on geomorphic formation in the typhoon area. Around Kudaka Island in the Ryukyu Islands, fringed coral reefs and the distribution of large reef blocks were investigated by aerial photo interpretation. Forty-one large reef blocks located on the reef flat and moat were measured and characterized. Movements of large reef blocks on them were observed from October 1994 to September 1996. Coral clasts, which were randomly collected from the moat, were placed on the reef flat under calm sea conditions and the direction and distance of movements were measured. Typhoon-induced waves swept across the reef edge and large blocks were transported from the reef slope to the reef flat. These blocks are classified as “groove blocks” and “spur blocks” based on their shape. The spur blocks, which originated from the reef framework and dead corals, are angular, while groove blocks are rounded as a result of abrasion by spur blocks. Spurs developed sideways during storms, and reef blocks were detached from spurs. Through aerial photo interpretation and field observations, it was determined that reef blocks on the reef flat were transported at intervals of a few decades. These movements are much quicker and the production of reef blocks much more frequent in the trade wind zone where blocks are transported on a geological time scale. Movements of blocks were observed during the winter seasonal wind and typhoons of 1996. In the summer of 1996, reef blocks were thrown up over 50m from the reef slope on the reef flat by the wave forces of typhoons with wind speed of 20m/s or more and maximum significant wave height of 6 m or more. Only groove blocks and live corals were transported and no movement of spur blocks was observed. Horizontal movements of large blocks were recorded for distances of over 50m and vertical movements of blocks were recorded for distances of over 5m during typhoons. Spur blocks would be transported by waves of typhoons bigger than those that occurred during this investigation. Reef slopes in the Ryukyu Islands are gentle, and therefore it is easy for reef boulders to be thrown up from reef slopes to reef flats. Most of blocks on the reef flat and the moat are characterized by depressions of erosion that suggest the blocks have been exposed to the air. The scattered reef blocks show a heterogeneous distribution in convex reef flats. This suggests that large blocks have been transported by wave processes. Blocks in the moat become live coral basements, and some blocks become cemented to the coral, causing the moat to pile up.
Article
The 2011 Tohoku Earthquake which exceeds the maximum magnitude of historical earthquake records could not be predicted. Which means, in the probabilistic tsunami hazard assessment for the future prediction, it is necessary to apply a different concept from the existing scenario tsunami assessment based on the maximum magnitude of historical earthquake tsunami. As a new modeling method for scenario tsunami, we propose a characterized tsunami source model which indicates setting method of the area of tsunami source and slip distribution by the inter-plate earthquake.
Article
The occurrence of large earthquakes and tsunamis along the Ryukyu Trench is a subject of continuing interest, the key to which is the long-term geological record. Here we describe the clast size and spatial distributions of similar to 2900 boulders on the reefs of the Ryukyu Islands, Japan, as markers of paleotsunamis and causative tsunamigenic earthquakes. Boulders of tsunami origin were observed only at a specific island group at the southern end, suggesting the local occurrence of tsunamigenic earthquakes there. In contrast, in the central to northern Ryukyu Islands, no evidence exists of tsunamis larger than those at the southern end of the Ryukyu Islands during the past 2000-3000 yr. These islands have numerous boulders deposited by storm waves during the past 2300 yr or earlier. Their spatial distribution has not been disturbed by large tsunamis. This suggests that large tsunamis did not strike this area during that period; nevertheless, these regions are seismically active. Our study shows that coastal boulder deposits present great potential to not only ascertain the histories and effects of paleotsunamis but also to constrain fault models of the causative earthquakes.
Article
Long-term activity of shallow very low frequency earthquakes (VLFEs) in the Ryukyu Trench, unassociated with recent large thrust earthquakes, was analyzed using a broadband seismometer network. The distribution of shallow VLFEs was divided into three large clusters. The activity of the VLFEs is modulated by repetitive slow slip events and the activity of these VLFEs increases to 2–3 times its ordinary rate at 10–20 days after the onset of the slow slip events. The activation of VLFEs could be generated by small increases in the order of approximately 0.1–0.5 kPa in Coulomb failure stress, suggesting that stress in the plate interface where the VLFEs occur is frequently released in small amounts. Moreover, the distribution of VLFEs is complementary to the historical tsunami source area and locked area. The distribution of the VLFEs indicates heterogeneity in interplate coupling along the trench.
Article
[1] The rupture parameters and magnitude of the AD 869 Jogan earthquake, a predecessor of the 2011 Tohoku earthquake, were previously estimated by matching tsunami deposit distributions with simulated inundation areas. The tsunami inundation associated with the 2011 Tohoku earthquake, however, extended farther inland than the sandy tsunami deposits. Numerical simulation of the 2011 tsunami indicated that flow depths and velocities were approximately 1 m and 0.6 m/s, respectively, at the most inland sand deposit sites on the Ishinomaki and Sendai plains. While these values depend on the assumed bottom roughness, we used these values to compare tsunami deposits and inundation simulation of the 869 Jogan earthquake from both uniform-slip and 2011-type variable-slip fault models. The results showed that the rupture length of the 869 Jogan earthquake was at least 200 km and its minimum moment magnitude was 8.6.
Article
We have conducted hydraulic experiments in an open channel with cubic and rectangular shaped solid blocks on the slope for investigating the boulder transport process by tsunami. In our experiments, the block was mainly seen to be transported by a bore due to rolling or saltation rather than by sliding. Previous models for the boulder transport by tsunamis assumed sliding as a mode of transport for the boulder. Therefore, these models underestimated the distance of the boulder moved by the tsunami when it was transported due to rolling or saltation. In this study, we have developed a practical model for the transport of a boulder by tsunami, which takes into account the various transport modes. We introduce an empirical variable coefficient of friction by assuming that the coefficient decreases with decrease in ground contact time when the block was transported by rolling or saltation. With the aid of this parameter, the model can explain various modes of transport, i.e., sliding, rolling, and saltation, and reproduces the experimental results well. We further applied this improved model to a tsunami boulder at Inoda area in Ishigaki Island, Japan, which was transported by the 1771 Meiwa tsunami. The calculated distance of transport of the boulder was approximately 650 m, which is consistent with the description in the historical document. Based on our calculations, we estimated hydraulic values of the tsunamis. Estimation of such hydraulic values is important for understanding the behavior and power of the historical tsunamis, besides aiding future disaster mitigation efforts.
Article
The study of palaeotsunamis preserved in the sedimentary record has developed over the past three decades to a point where the criteria used to identify these events range from well-tested and accepted to new methods yet to receive wide application. In this paper we review progress with the development of these criteria and identify opportunities for refinements and for extending their application to new settings. The emphasis here is on promoting the use of multiple proxies, selected to best match the context of the site or region of interest. Ultimately, this requires that palaeotsunami research must be a multidisciplinary endeavour and indeed, extend beyond the geological sciences of sedimentology and stratigraphy and, to include knowledge and approaches from field such as archaeology, anthropology and sociology. We also argue that in some instances, despite the use of multiple proxies, the evidence for tsunami inundation of a coast simply may not be preserved.
Article
On 29 September 2009 a large tsunami struck the US territory of American Samoa, killing 34 people. However, had it not been for the island's tsunami preparation, the death toll would have been much higher. An integral part of preparing for a tsunami is knowing how often large tsunamis occur, knowledge that can be improved by using paleo-tsunami sediment deposits to lengthen the historical tsunami record. Doing so requires a detailed understanding of the processes that control sediment transport during tsunamis and thus the patterns of deposition, which may vary substantially in sediment limited and sediment rich environments. As American Samoa is relatively young and composed of volcanic islands surrounded by coral reefs, sandy beaches occur primarily as pocket beaches between volcanic rock headlands. The extent and depth of littoral sediment available for transport is therefore limited. This is in contrast to previous studies which have focused primarily on coastal environments with extensive sand supply. Here, field observations and a numerical model are used to investigate tsunami-induced sediment transport in two sediment limited embayments (Massacre and Fagafue Bays) on the north side of the island of Tutuila in American Samoa. Detailed measurements of bathymetry, topography, tsunami flow depth and direction, sediment deposition, reef and vegetative roughness, and the extent of sediment available for transport were collected approximately two weeks after the tsunami. Both embayments contain sandy sediments that extend all the way (Massacre Bay) and part of the way (Fagafue Bay) along the beach at the head of the embayment. The observed onshore sediment deposit created by the tsunami was limited and patchy in both embayments even though a number of large coral boulders were transported onshore at both locations. The extent to which the limited supply of sediment plays in producing the patterns of deposition observed in these two bays is examined using a three-dimensional coupled hydrodynamic/sediment transport/morphological change model (Delft3D). To isolate this effect, model simulations examine the effects on sediment deposition of wave focusing owing to embayment shape and reef channels, and the strong return flow generated by the steep onshore topography. Differences between the depositional patterns in sediment limited and sediment rich environments are identified by comparing the results from these simulations with results from simulations where the extent and depth of the sediment source are increased and from simulations conducted in a sediment rich environment (Kuala Meurisi, Sumatra).
Article
Field observations and numerical simulations are used to explore tsunami inundation and sediment transport in an embayment (Fagafue Bay) on the north side of Tutuila, American Samoa during the 29 September 2009 South Pacific tsunami. Field observations of the nearshore bathymetry and topography, tsunami flow depth and sediment deposition, and extent of movable sandy sediment remaining on the beach were collected during two field surveys approximately two and five weeks after the tsunami. Onshore measurements of flow depth at forty-eight locations indicate the wave inundated almost 250m onshore with a depth exceeding 7m locally. The tsunami deposited patchy areas of sediment up to 0.2m thick interspersed with a thin dusting (
Article
The 24 April 1771 Yaeyama earthquake generated a large tsunami with a maximum runup of 30 m, causing significant damage in south Ryukyu, Japan, despite the weak ground shaking. Previously proposed mechanisms of the tsunami include intraplate faulting or submarine landslide in the forearc slope. In this study, I estimate the fault parameters of the 1771 earthquake by numerically computing the tsunami heights and comparing them with the recorded heights. The result indicates that the source fault of the tsunami is very close to the Ryukyu Trench. The results are consistent with a thrust-faulting earthquake that had a fault-width of less than 50 km. The 1771 Yaeyama tsunami was caused by a tsunami earthquake (Mw = 8.0) that occurred in the subducted sediments beneath the accretionary wedge.
Article
The amount of shallow seismic activity in subduction zones varies greatly from region to region. This paper quantifies this seismicity by calculating seismic moment release rates and seismic slip rates for 24 subduction zones. A time history plot of the seismic moment release is presented for each subduction zone; these exhibit the differences in the seismic release patterns. The moment release rates are compared with various subduction parameters in order to determine which factors influence the degree of coupling. These parameters include the age of the subducting lithosphere, absolute velocities of the upper and subducting plates, convergence velocity, and length maximum depth, and dip of the Wadati-Benioff zone. The moment release rate decreases as the age of the subducting lithosphere increases, when the zones belonging to a single subducting plate are considered. It is suggested that within a single plate, the age is the dominating factor affecting the strength of seismic coupling but that each plate as a whole has a characteristic moment release budget. Zones with retreating upper plates tend to have lower moment release rates. The moment release rate does not increase with convergence velocity; no simple relationship was found between these two parameters.
Article
Cores from Cascadia deep-sea channel contain sequences of turbidites that can be correlated and dated by the first occurrence of volcanic glass from the Mount Mazama eruption (6845±50 radiocarbon yrBP). Turbidity currents from the tributaries appear to have occurred synchronously to form single deposits in the main channel, there being only 13 turbidite deposits in the lower main channel since the Mazama eruption, instead of the twice as many expected if the tributaries had behaved independently. In addition to the Cascadia Channel, 13 post-Mazama turbidites have been deposited in the Astoria Canyon and at two sites off Cape Blanco, sample locations that span 580 km of the Oregon-Washington margin. Pelagic intervals deposited between the turbidites suggest that in each place the turbidity currents occurred fairly regularly, every 590±170 years on average. The best explanation of the spatial and temporal extent of the data is that the turbidity currents were triggered by 13 great earthquakes on the Cascadia subduction zone. The variability of turbidite timing is similar to that for great earthquake cycles. The thickness of the topmost pelagic layer suggests the last event was 300±60 years ago (from three places along the margin), but this number may be a biased underestimate. It is, however, consistent with the youngest sudden-subsidence event on the Washington coast. The turbidite data demonstrate that the near-term hazard of a great earthquake on the Cascadia subduction zone is of the order of 2-10% in the next 50 years.
Article
Case studies of recent tsunami impacts have proven to be extremely useful in understanding the geologic processes involved during inundation and return flow, and refining the criteria used to identify paleotsunami deposits in the geologic record. Here, we report on erosion, deposition and associated landscape change resulting from the March 11, 2011 Tohoku-oki tsunami along a nearly 4.5 km shore-normal transect on the coastal plain near Sendai, Japan. The study area on the broad, low-relief prograding coastal Sendai plain comprised a sand beach backed by ~ 3 m high sand dunes and a forest, a wetland, the Teizan canal, agricultural rice fields, buildings and roads.
Article
We report initial results from our recent field survey documenting the inundation and resultant deposits of the 2011 Tohoku-oki tsunami from Sendai Plain, Japan. The tsunami inundated up to 4.5 km inland but the > 0.5 cm-thick sand deposit extended only 2.8 km (62% of the inundation distance). The deposit however continued as a mud layer to the inundation limit. The mud deposit contained high concentrations of water-leachable chloride and we conclude that geochemical markers and microfossil data may prove to be useful in identifying the maximum inundation limit of paleotsunamis that could extend well beyond any preserved sand layer. Our newly acquired data on the 2011 event suggest that previous estimates of paleotsunamis (e.g. 869 AD Jōgan earthquake and tsunami) in this area have probably been underestimated. If the 2011 and 869 AD events are indeed comparable, the risk from these natural hazards in Japan is much greater than previously recognized. Keywords: Tsunami deposits; Tohoku-oki tsunami; Japan
Article
Abstract The taxonomic diversity of hermatypic corals decreases with increasing latitude, which correlates with sea-surface temperatures. However, little is known about latitudinal changes in the taxonomic diversity and biogeographic patterns of larger benthic foraminifera, although their physiological requirements are similar to those of hermatypic corals because of their symbiotic relationships with microalgae. The present study examined how the abundance and taxonomic composition of larger foraminiferal assemblages in shallow-water reef sediments change with latitude along the Ryukyu Islands (Ryukyus), which are located near the northern limit of coral-reef distributions in the western Pacific Ocean. Three islands from different latitudes in the Ryukyus were selected to investigate latitudinal changes in larger foraminiferal assemblages: Ishigaki Island (24°20′N, 124°10′E), Kudaka Island (26°09′N, 127°54′E) and Tane-ga-shima Island (30°20′N, 131°E). Four sediment samples were taken at each of three topographic sites (beach, shallow lagoon and reef crest) on the reef flat of each island. Foraminiferal tests of a 2.0- to 0.5-mm size fraction were selected, identified and counted. The variations in foraminiferal abundance in reef sediments from three latitudinally different islands exhibit two contrasting trends along reef flats: a shoreward decrease on Ishigaki and Tane-ga-shima Islands and a shoreward increase on Kudaka Island. A total of 25, 24 and 13 foraminiferal taxa were identified in Ishigaki, Kudaka and Tane-ga-shima Islands, respectively. Baculogypsina sphaerulata, Neorotalia calcar and Amphistegina spp. were dominant (i.e. >3% of foraminiferal assemblages) in the three islands. Calcarina gaudichaudii and Calcarina hispida were common on Ishigaki and Kudaka Islands but were absent on Tane-ga-shima Island. Larger foraminiferal assemblages from three different reef-flat environments on Ishigaki Island can be distinguished, whereas those from the three environments on Kudaka and Tane-ga-shima Islands are similar in composition. These latitudinal changes in larger foraminiferal assemblages in reef sediments may possibly be caused by variations in the topography of reef flats, distributions and standing crops of living foraminifers on reef flats, and the northern limit of some calcarinid species in the northern Ryukyus.
Article
Tsunamis are high energy events capable of transporting extremely heavy loads including boulders. We compare boulder deposits created by two modern tsunami events, the 2004 Indian Ocean and the 2009 South Pacific tsunamis, where the boulder sources were in similar topographic settings, and for which we have accurate data on the wave characteristics. Boulder distribution, preferential orientation and numerical simulation of boulder transport are discussed. A comparison between the impacts of the South Pacific and Indian Ocean tsunamis shows similar characteristics, such as limited landward extent and the absence of landward fining. Differences between the results from modelling and field data are most probably caused by variables such as coastal plain roughness (buildings, trees), microtopography, particle shape, and boulder collision during transport that are summarised as coefficients in the mathematical models. Characterising modern events through coarse sediment deposits provides valuable information to help identify and interpret palaeo-tsunami imprints on coastal landscapes.
Article
A combination of numeric hydrodynamic models, a large-clast inverse sediment-transport model, and extensive field measurements were used to discriminate between a tsunami and a storm striking Anegada, BVI a few centuries ago. In total, 161 cobbles and boulders were measured ranging from 1.5 to 830kg at distances of up to 1km from the shoreline and 2km from the crest of a fringing coral reef. Transported clasts are composed of low porosity limestone and were derived from outcrops in the low lying interior of Anegada. Estimates of the near-bed flow velocities required to transport the observed boulders were calculated using a simple sediment-transport model, which accounts for fluid drag, inertia, buoyancy, and lift forces on boulders and includes both sliding and overturning transport mechanisms. Estimated near-bed flow velocities are converted to depth-averaged velocities using a linear eddy viscosity model and compared with water level and depth-averaged velocity time series from high-resolution coastal inundation models. Coastal inundation models simulate overwash by the storm surge and waves of a category 5 hurricane and tsunamis from a Lisbon earthquake of M 9.0 and two hypothetical earthquakes along the North America Caribbean Plate boundary. A modeled category 5 hurricane and three simulated tsunamis were all capable of inundating the boulder fields and transporting a portion of the observed clasts, but only an earthquake of M 8.0 on a normal fault of the outer rise along the Puerto Rico Trench was found to be capable of transporting the largest clasts at their current locations. Model results show that while both storm waves and tsunamis are capable of generating velocities and temporal acceleration necessary to transport large boulders near the reef crest, attenuation of wave energy due to wave breaking and bottom friction limits the capacity of storm waves to transport large clast at great inland distances. Through sensitivity analysis, we show that even when using coefficients in the sediment-transport model which yield the lowest estimated minimum velocities for boulder transport, storm waves from a category 5 hurricane are not capable of transporting the largest boulders in the interior of Anegada. Because of the uncertainties in the modeling approach, extensive sensitivity analyses are included and limitations are discussed. KeywordsBoulder–Tsunami–Hurricane–Cyclone–British Virgin Islands–Caribbean
Article
This study investigates the size, position and the long axis orientation of 210 boulders at Kudaka Island, Japan. These boulders were deposited from the reef crest to the slope of the back reef moat, distributed within 275 m from the reef edge. Most boulders were rectangular to ellipsoidal, without sharp broken edges. They are reef rock fragments estimated as < 63 m3 (< 127 t). The second largest boulder (54 t) was not observed in aerial photographs taken in 1977 and 1993, although it appears in photographs taken in 2005 and 2007. Considering that no large tsunami event occurred during 1993–2005, the second largest boulder is expected to have been emplaced by typhoon-generated storm waves. Moreover, the positions of many boulders were found to have shifted after 1977. These boulders were highly likely to have been repositioned by the storm waves. Results showed that boulders' motion follows an exponential fining trend shoreward. This trend fits well with the distribution of the height of the storm wave after breaking on the reef flat. The largest storm waves after 1977 (typhoon 0704 in 2007) were probably responsible for the current boulder distribution. Using the relation between the distributions of boulders and the significant wave height of typhoon 0704, the approximate transport distance of boulders by an arbitrary storm wave at the island can be estimated. The storm wave boulders' distribution is also useful to estimate the storm wave properties: we estimated the maximum current velocity distribution of waves generated by typhoon 0704 on the reef flat as up to 6.5 m/s using the boulder distribution.
Article
Coastal boulder accumulations are often mentioned in the literature, even though their interpretation remains difficult, especially along rock coasts affected both by storms and tsunamis. Studies on the geomorphic impact of such high-energy events are actually of great interest, since their intensity and frequency are key issues for the future evolution of coasts in the framework of the global change. The southwest coast of Iceland faces the powerful storms of the North Atlantic Ocean, with wave heights of more than 15 m. The probability for past and present tsunamis to hit this coast is very low. In this paper, we describe boulder accumulations along the volcanic rock coast of Reykjanes (southwest Iceland). They consist of cliff-top boulders, clusters and ridges, beaches, and boulder fields. Large boulders, up to 70 t in weight, have been transported and deposited up to 65 m inland (6 masl). The maximum limit of boulder deposition and driftwood was found respectively 210 m and 550 m inland. Storms appear to be a predominant factor in the geomorphic evolution of Reykjanes coasts. Our observations also give new insight for the interpretation of coastal boulder accumulations. Processes of erosion and deposition by tsunamis are a rising topic in the literature, and the effects of recurrent and powerful storms are neglected.
Article
Using a GPS derived velocity field, we try to estimate the kinematic features of the Ryukyu arc, and discuss its implications to the spreading of the Okinawa trough. The GPS derived strain rate field implies that internal deformation within the Ryukyu arc is not significant, therefore, we adopt a rigid block rotation model for the motion of the Ryukyu arc. The velocity field in the Ryukyu arc is best explained by a model with three blocks rather than one with only one block. The estimated block rotations predict the spreading along the Okinawa trough. The predicted spreading is slow (10 mm per year) in the north and fast (50 mm per year) in the southwest. The direction of the predicted spreading is normal to the Okinawa trough (N170°E–N180°E) in the southwest, while in the north the direction of the spreading is oblique to the Okinawa trough (N150°E–N180°E).
Article
The tsunami of 2004 in the Indian Ocean transported thousands of meters-long boulders shoreward at Pakarang Cape, Thailand. We investigated size, position and long axis orientation of 467 boulders at the cape. Most of boulders found at the cape are well rounded, ellipsoid in shape, without sharp broken edges. They were fragments of reef rocks and their sizes were estimated to be < 14m3 (22.7t). The distribution pattern and orientation of long axis of boulders reflect the inundation pattern and behavior of the tsunami waves. It was found that there is no clear evidence indicating monotonous fine/coarse shoreward trends of these boulders along each transect line. On the other hand, the large boulders were deposited repeatedly along the three arcuate lines at the intertidal zone with a spacing of approximately 136m interval. This distribution pattern may suggest that long-lasting oscillatory flows might have repositioned the boulders and separated the big ones from small. No boulders were found on land, indicating that the hydraulic force of the tsunami wave rapidly dissipated on reaching the land due to the higher bottom friction and the presence of a steep slope. We further conducted numerical calculation of tsunami inundation at Pakarang Cape. According to the calculation, the sea receded and the major part of the tidal bench (area with boulders at present) was exposed above the sea surface before the arrival of the first tsunami wave. The first tsunami wave arrived at the cape from west to east at approximately 130min after the tsunami generation, and then inundated inlands. Our calculation shows that tsunami wave was focused around the offshore by a small cove at the reef edge and spread afterwards in a fan-like shape on the tidal bench. The critical wave velocities necessary to move the largest and average-size boulders by sliding can be estimated to be approximately 3.2 and 2.0m/s, respectively. The numerical result indicates that the maximum current velocity of the first tsunami wave was estimated to be from 8 to 15m/s between the reef edge and approximately 500m further offshore. This range is large enough for moving even the largest boulder shoreward. These suggest that the tsunami waves that were directed eastward, struck the reef rocks and coral colonies, originally located on the shallow sea bottom near the reef edge, and detached and transported the boulders shoreward.
Article
Sedimentary features and identification criteria of boulders deposited by tsunamis and storm waves are highly controversial because of the lack of detailed studies of boulders that are known to have been deposited by tsunami or storm waves. The coastal boulder fields of the Ryukyu Islands, Japan are one of the few places where comparisons can be made between the distribution and characteristics of boulders deposited by a known historical tsunami and storm waves. The 1771 Meiwa Tsunami struck the southern Ryukyu Islands (Miyako–Yaeyama Islands) and reliable historical documents describe run-up heights of up to 30 m. The displacement of specific boulders by the tsunami is also described in detail. Some of the islands away from the Miyako–Yaeyama Islands were unaffected by this tsunami, but they have been extensively affected by typhoon-generated storm waves. On these islands, the boulders were commonly deposited on the reef flat within 300 m of the reef edge as an exponentially fining landward deposit. This provides a useful indication of the transport limit for storm waves on the Ryukyu Islands. In the tsunami-affected islands, boulders of different types have been deposited both on the reef crest and along the shoreline. The reef crest boulders are identified as storm wave emplaced, whereas those along the shoreline are interpreted as tsunami boulders (“tsunami-ishi” in Japanese) because they are exceedingly heavy and are deposited well beyond (ca. 1.5 km from the reef edge) the transport limit for storm waves. Their 1771 Meiwa Tsunami origin is supported by 14C age results, although prior tsunami(s) may have deposited some of the boulders. Based on these results, we infer that the difference between the wave periods of tsunami and storm waves is crucial to differentiating tsunami boulders from other enigmatic boulder deposits around the world. Differences in wave period are reflected in differences between the spatial and clast size distributions of boulder deposits. The distribution and sedimentary characteristics of tsunami boulders therefore provide useful data for estimating possible tsunami sources. The boulders on the Ryukyu Islands are also useful for differentiating between tsunami and storm wave emplacement and for estimating their hydrodynamic properties.
Article
Few studies have been conducted on modeling boulder transport by tsunamis despite considerable research on the analysis of boulder deposits. A detailed description of the derivation of governing equations for boulder transport in submerged, partially submerged, and subaerial (not in contact with fluid) is presented, and then a numerical model is proposed to solve the governing equations in one dimension. Subsequently, the model is used to analyze the transport of calcareous boulders detached from a seawall in Lhok Nga (northwestern Sumatra, Indonesia) by the 2004 Indian Ocean tsunami. A few simulated transport distances match field observations, but the others are higher than the field measurements. Clast-to-clast interactions at the inception of transport would have a major impact on changes in transport distance, dissipating the energy in impulses as destruction of the seawall releases different sizes of boulders with different velocities. Moreover, surface microtopographical effects could completely stop the transport prematurely. The difference between the simulated results and the field observations is partly attributed to limitations of the numerical model. No landward fining was observed in the field measurements, but numerically predicted results showed a reasonable trend of landward fining.
Invasion of about 3400 cal BP large wave in the southeastern Okinawa Island and the surroundings, the Ryukyus, Japan, as deduced from coralline deposits
  • Kawana
Kawana, T., 2006. Invasion of about 3400 cal BP large wave in the southeastern Okinawa Island and the surroundings, the Ryukyus, Japan, as deduced from coralline deposits. Ryukyu Univ. Kyouikugaku Bukiyo 68, 265-271 (in Japanese).
Estimation of the historical large wave based on the position of coral boulders on the reef
  • Nakaza
Formative history of coral reef and earthquake and tsunami at Ryukyu Islands
  • Kawana
Kawana, T., 2011. Formative history of coral reef and earthquake and tsunami at Ryukyu Islands. Ryukyu Islands Dur. Prehist. Hist. Age, 63-86 (in Japanese).